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Color reference files read by Imatest can be CSV or CGATS format containing L*a*b* color and luminance reference data.
The reference illuminant is assumed to be the same as the selected color space (D65 for sRGB, etc.). This is at variance with current practice, where used to create L*a*b* files are assumed to have D50 data. The new LAB D50 file (Imatest 4.4+) is recommended because it will transform the reference values using a Bradford Transform to move them to the selected color space’s white point. This is the recommended approach if you have spectrophotometer measurements for your individual chartreference files is almost always D50. The most widely used color spaces (sRGB and Adobe RGB) have D65 white points5. A Bradford Transform is applied to convert the D50 file data to D65 color spaces. It is applied to spectrophotometer measurements for individual charts.
Color Reference files
Color CSV reference files files consist of the number of lines that the test chart has patches, with L*, a*, b* values on each line separated by spaces, commas (,), or semicolons (;). Example (first 3 lines of 24):
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Color reference files can also be derived from measured values from a camera you determine to be a “gold standard” by running Color/Tone Interactive, then pressing File, Save L*a*b* results as CSV reference (making sure to check CSV reference file of L*a*b* results…). This can be useful for manufacturing testing because the camera under test is compared with the attainable values of a golden sample rather than unattainable ideal values.
The CGATS file format is also supported.
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Color/Tone Interactive General mxn charts (4×12 shown on the left). Any chart with an m x n grid can be analyzed. You must specify the grid size (click Settings, ISO speed & mxn chart settings to set the number of rows (m) and columns (n). You must also enter a CSV or CGATS file with reference patch settings (typically L*, a*, b* values, one set per line). If you have a spectrophotometer you can create a custom reference file (CGATS format) using techniques described in Measuring test chart patches.
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Colorcheck Reference File (legacy module: Color/Tone is recommended for new work)
Measuring Test Chart Patches with a Spectrophotometer for details about how to create a reference file using measurements from an instrument.